1. Field of the Invention
This invention pertains to semiconductor packaging in general, and in particular, to a thin, leadframe-type of semiconductor package having an improved heat dissipating capability, improved resistance to penetration by moisture, and an improved down-bond capability by virtue of an enhanced leadframe therefor.
2. Description of the Related Art
Leadframe types of semiconductor packages are well known and widely used throughout the electronics industry to house, mount, and interconnect a variety of types of integrated circuits ("ICs"). An IC is typically formed on a single die, or "chip," that is cut from a semiconductor wafer containing a large number of identical dies. The dies themselves are relatively small and fragile, and are susceptible to harmful environmental elements, particularly moisture, and accordingly, must be packaged in affordable, yet robust, packages capable both of protecting them and permitting them to be reliably mounted to, for example, a printed circuit board ("PCB") and interconnected with associated electronic components mounted thereon.
One type of well known and widely used low-profile leadframe IC package is the so-called "thin shrink small outline package," or "TSSOP," which typically includes a plurality of leads on each of two sides of a thin, rectangular body. Other types of packages employing leadframes having leads on either two or all four sides of a rectangular body include "lead-on-chip" ("LOC"), "chip-on-lead" ("COL"), "small outline integrated circuit" ("SOIC"), "plastic dual in-line package" ("PDIP"), "shrink small outline package" ("SSOP"), "plastic leaded chip carrier" ("PLCC") and "quad flat package" ("QFP") packages.
A representative conventional leadframe 10 and two alternative semiconductor package 12 and 12' made from it are illustrated in FIGS. 5-8. The conventional leadframe 10 typically includes a plurality of electrically conductive leads 14 that are temporarily held together in a planar arrangement about a central opening 16 during package manufacture by a plurality of expendable longitudinal and lateral tie-bars 18 that form a rectangular frame enclosing the leads. A semiconductor die mounting pad 20 is supported within the central opening 16 by one or more die pad support leads 22. The leads 14 extend from a first end 26 integral with the rectangular frame to an opposite second end 24 adjacent to, but spaced from, the central opening 16. The longitudinal tie-bars 18 may include tooling or sprocket holes 28 for accurately positioning and/or advancing the leadframe during the package manufacturing process.
The conventional leadframe 10 is typically die-stamped from a sheet of flatstock metal, such as a copper or aluminum alloy, typically about 0.125-0.250 millimeters ("mm") in thickness, and may be deployed in the form of a strip of identical, interconnected leadframes, such as those illustrated in FIGS. 5 and 6, for either the sequential or simultaneous fabrication of a plurality of packages thereon.
During package manufacture, an IC die 30 is attached to the die pad 20, typically by solder, a layer of adhesive 32, or a double-sided adhesive tape. After the die is attached to the pad, wire-bonding pads 34 on top of the die are electrically connected to corresponding ones of the inner ends 24 of the leads 14 by fine, conductive bonding wires 36 to connect power, ground, and signals between the die and the leads. Additionally, some of the pads 34 that serve a grounding function may also be "down-bonded" to the die pad 20 by other conductive bonding wires 38 to ground the die to the die pad.
When wire-bonding is complete, each of the bonded assemblies is placed between the halves of a clam-shell mold (not illustrated) and a protective envelope 40, typically of a high density epoxy resin, is molded, usually by transfer-molding, over the assembly to enclose and seal the die 30, the inner ends 24 of the leads 14, and the wire bonds 36 and 38 against harmful environmental elements (see FIGS. 7 and 8). After molding, the temporary tie-bars 18 are cut away from the package 12 or 12' and discarded, their function having been assumed by the rigid epoxy envelope 40, and the outer ends 26 of the leads 14 are left exposed by the envelope 40 for interconnection of the package with other, associated circuitry (not illustrated).
A problem increasingly encountered in the semiconductor packaging industry today relates to the amount of heat experienced by the device during manufacture and assembly, as well as that generated by the device during operation, and the ability of the package to spread that heat uniformly and dissipate it to the environment effectively. As electronic devices grow more compact, yet faster and more functional, the problem increasingly becomes one of getting rid of more heat from packages that are the same, or increasingly, smaller in size, and this is generally the case not only for leadframe types of packages, but others as well.
It may be noted in the conventional leadframe package 12 illustrated in cross-section in FIG. 7 that the support leads 22 that support the die pad 20 within the central opening 16 have been given a "down-set," i.e., angled downwardly, such that the die pad 20 is vertically displaced below the plane of the leads 14. This down-set places the die 30 closer to the bottom of the package. Since the thermal resistance between the die and a heat-sinking surface 42 to which the package is mounted is proportional to the thickness of the material between them, this down-set reduces that resistance, thereby affording the package 12 a greater heat dissipating capability than packages without such a down-set.
The down-set die pad also provides two other, non-thermal benefits; namely, it reduces the overall height of the package 12, which is of interest to package designers faced with a requirement for thinner, more compact packages, and it also reduces the length of the bonding wires 36 extending between the die 20 and the leads 14 of the leadframe. In some applications, such as high-frequency and/or high-power applications, this reduction in conductor length improves the electrical performance of the packaged device.
The leadframe package 12' illustrated in FIG. 8 shows an alternate, "deep down-set" die pad embodiment in which the bottom surface of the die pad 20 is exposed through the bottom surface of the epoxy envelope 40. This not only further reduces the height of the package 12', the length of the bonding wires 36, and the thermal resistance between the die 30 and the surface 42 of a heat sink, but also enables the die pad 20 to be thermally coupled more directly to the heat sink surface by an efficient conductor of heat, for example, by a layer 44 of solder or a thermally conductive adhesive. Thus, this structure might be thought to represent an optimum solution for the problem of dissipating heat from a small outline, low-profile leadframe package. However, for the reasons discussed below, this configuration also creates some packaging problems that, to a large extent, offset the thermal benefits that it yields.
One of these relates to the resistance of the package to penetration by harmful moisture. It may be noted in the prior-art leadframe package 12' having an exposed die pad 20 shown in FIG. 8 that a seam 46 is defined at the interface of the die pad and the epoxy plastic envelope 40. Since a perfect adhesion between the die pad and the envelope along the entire length of the seam 46 is impractical, the seam may define the locus of one or more microscopic cracks for the entry of moisture.
So long as the moisture does not reach the die 30, this does not present an immediate problem. However, it does create a longer-term problem with repeated high-low temperature cycling of the device, in that any moisture trapped in the cracks at a low temperature will vaporize, and hence, expand, at a high temperature of the device, causing the microscopic cracks to open further and propagate around the edge of the die pad and more deeply into the package, until the cracks eventually reach the die, at which point the device can be rendered defective by moisture contamination of the die due to possible corrosion of metallizations and subsequent current leakage through the corrosive path.
Thus, the prior art leadframe package 12 without an exposed die pad illustrated in FIG. 7 exhibits a better resistance to penetration by moisture than does the package 12' illustrated in FIG. 8, although the latter exhibits a superior ability to dissipate heat from the die, for the reasons discussed above. Also, generally speaking, for packages that include an exposed die pad, the greater the length of the path between the die and the external boundary between the die pad and the plastic envelope, the greater is the resistance of the package to penetration by moisture over time. Therefore, while it is desirable on the one hand to position the die as closely as possible to a surface of the package for good heat dissipation, it is desirable on the other hand to position the die as remotely as possible from a surface of the package for good resistance to its penetration by moisture.
Another problem of conventional leadframe packages relates to the "down-bonds" 38 described above, i.e., the wire bonds made between the grounding pads on the die 30 and the upper surface of the die pad 20 to ground the die. As may be seen in the cross-sectional views of FIGS. 7 and 8, the down-bonds are made on a margin of the die pad that extends laterally beyond the edges of the die. The die is typically attached to the die pad by solder, or by a layer 32 of an adhesive, which is typically applied as a liquid. For manufacturing reasons, it is typical in either case for the attachment layer 32 to flow out on the surface of the die pad beyond the edges of the die, thereby preventing the down-bonds from being made immediately adjacent to the die, and necessitating their being made further outboard of it. This, in turn, results in an undesirable, and in some high-frequency applications, unacceptable lengthening of the down-bonded grounding wires 38. Accordingly, it is desirable to have a leadframe design that permits the down-bonded wires 38 to be made as short, and hence, as close to the edge of the die, as possible.
Another problem with prior-art leadframe packages also relates to the down-bonds, and the amount of residual shear stress present on the top surface of the die pad upon which the down-bonds are made. As described above, an epoxy resin envelope 40 is typically transfer-molded, at a relatively high temperature, e.g., 175-200.degree. C., onto the leadframe after the die is wire-bonded to it. As a consequence of the relatively large differences in the respective thermal coefficients of expansion of the various materials of the package, e.g., epoxy resin, silicon, copper, aluminum, and the like, a widely-varying distribution of residual shear stresses is established at different locations within the package when it is cooled to room temperature.
This distribution of residual shear stress is very complex and includes so-called "neutral regions" where the shear stress is negligible and/or changes sign, and is closely dependant on the particular design of the package and the materials contained within it. However, certain generalizations can be made, including that the residual shear stress acting on the top surface of the die pad of a deep down-set die pad, and hence, on the down-bonds made thereat, is significantly greater than that acting on the wire bonds made to the leads of the package, because the latter wire bonds are made in a region of the package that is closer to the radius of curvature of the package, i.e., a neutral region where the residual shear stress is negligible.
As a result of the greater level of shear stress acting upon them, the down-bonds, which, like the wire bonds to the leads of the package, are typically ultrasonic "stitch," or thermosonic "crescent," bonds, are subject to a much higher incidence of de-lamination from the die pad, i.e., bond failure, than the bonds made to the leads. And, as above, while it is possible to reduce the level of shear stress on the down-bonds by moving the top surface of the die pad closer to the plane of the leads, this has the retrograde effect of moving the die further away from the surface of the heat sink, and hence, increasing the thermal resistance between the two.
Therefore, there is need for a thin, affordable, small-outline leadframe semiconductor package that provides an enhanced heat dissipating capability, an improved resistance to penetration by moisture, and a down-bond region immediately adjacent to the edges of the die that minimizes the length of down-bonded wires and subjects the down-bonds to a minimum of residual shear stress.